A multi-hop wireless network is receiving attention, which relays communication between terminals in a wireless environment to make it possible for a wireless terminal to communicate with a remote base station. However, with the multi-hop wireless network using a single channel, there is a problem that throughputs vary for wireless terminals whose number of hops from a base station differ from each other.
That is, the throughput is lower for a wireless terminal whose number of hops from a base station is greater than for a wireless terminal whose number of hops from a base station is less. The reason is that IEEE802.11MAC protocol generally used in the wireless terminal link layer takes only a single-hop communication into consideration. In the single-hop communication, the timing of transmission and reception between terminals capable of sensing each other is properly controlled by a communication control called RTS/CTS. However, in a multi-hop communication where not all the terminals can be sensed, it becomes hard to maintain fairness of transmission right among terminals due to problems of the protocol.
FIG. 6 shows a case where the fairness of transmission right among terminals is maintained by RTS/CTS, and FIG. 7 shows a case where the fairness of transmission right among terminals is not maintained by RTS/CTS. In FIGS. 6 and 7, solid-line circles indicate respective wireless terminals, broken-line circles indicate communication ranges of the respective terminals, and arrows indicate the transmission direction of data. The numerals in the solid-line circles indicate the terminal number.
FIG. 6 shows a case where terminal 1 and terminal 2 performs transmission to terminal 0. In this case, for example, at the time of transmission by terminal 1, a Request-To-Send (RTS) signal is transmitted to terminal 0 before the transmission, and a Clear-To-Send (CTS) signal is transmitted from terminal 0 that received the RTS signal. When the CTS signal is received, except for the terminal that transmitted the RTS signal, none of the terminals within the communication range of terminal 0 performs transmission during the transmission by terminal 1, and thus, fairness among the terminals is maintained.
On the other hand, in the example shown in FIG. 7, RTS/CTS control is performed as in the case shown in FIG. 6 before transmission from terminal 1 to terminal 0, but the CTS signal from terminal 0 does not reach terminals other than terminal 1. However, transmission data from terminal 1 reaches terminal 2, and when terminal 3 simultaneously transmits an RTS signal to terminal 2 to perform transmission, collision occurs, and terminal 2 cannot receive the RTS signal from terminal 3.
Thus, terminal 3 retransmits the RTS signal. However, the waiting time at this time, that is a so-called back-off time, is longer than the usual back-off time, and thus, the probability of terminal 3 obtaining the transmission right is reduced compared to those of terminal 1 and terminal 2.
In a multi-hop wired network, as a means for improving the fairness of throughput among terminals, there is a round-robin scheduling that provides a relay terminal with a queue for each flow. However, in the multi-hop wireless network, since the fairness of transmission right among terminals is not maintained as described above, the fairness cannot be achieved with the simple round-robin scheduling. On the other hand, if transmission is suspended until packets arrive from respective flows, almost complete fairness can be achieved. However, if the transmission rate of each terminal differs from each other, the transmission is performed at the lowest transmission rate, and thus, there is a problem that the communication bandwidth is not efficiently used.
For example, a technology is described in Patent Document 1, which resolves the problem of a throughput gap resulting from the difference in hop count numbers in a multi-hop wireless network by determining packet size based on hop count information of each packet.
[Patent Document] JP-A-2003-273788